1. Field of the Invention
The present invention relates to a magnetic resonance imaging apparatus and a magnetic resonance imaging method which excites nuclear spins of an object with a RF (radio frequency) signal having the Larmor frequency and reconstructs an image based on a MR (magnetic resonance) signal generated in response to the excitation, and more particularly, to a magnetic resonance imaging apparatus and a magnetic resonance imaging method which make it possible to reduce positional shift of an image due to frequency correction of an excitation pulse with a frequency offset.
2. Description of the Related Art
MRI (magnetic resonance imaging) is an imaging method that excites atomic nuclei spins of objects put in a static magnetic field by RF signals having a Larmor frequency and then reconstructs an image from MR signals which occur in response to this excitation.
Up to now, in the MRI field, shimming to adjust uniformity of a static magnetic field is performed to lower degradation of image quality caused by influence of non-uniformity in the static field. And, a resonant frequency of protons that are included in an imaging cross-section in the static magnetic field adjusted by shimming is obtained by a pre-scan and a frequency offset to set a previously adjusted center frequency of an RF excitation pulse to a resonant frequency according to an imaging cross section. Additionally, on a scan for diagnostic imaging performed following the pre-scan, an RF excitation pulse center frequency is shifted by the frequency offset and used for excitation of protons (for example, refer to Japanese Patent Application (Laid-Open) No. 7-327960).
Techniques of magnetic resonance imaging are also known where, to get a large FOV (field of view) in the direction of a moving patient supporting bed table, the table may be moved continuously while imaging. A similar stepping-table method is known as well to perform 3D (three-dimensional) imaging by moving the bed table by steps while an injection of contrast medium provides images to be projected (for example, refer to Japanese Patent Application (Laid-Open) No. 8-71056 and Japanese Patent Application (Laid-Open) No. 2002-95646). These techniques can be used for the case of imaging a wide area which cannot be imaged all at once e.g., imaging of the whole body). Plural images which are collected in conjunction with moving the bed are combined with each other by compound processing. By this, an image of the wide area can be obtained.
In recent years, in the field of magnetic resonance imaging, a scan under EPI (Echo Planar Imaging) method collects plural echo signals sequentially by sequentially inverting a gradient magnetic field at a high speed after one excitation of nuclear resonance. However, using this EPI method, if an excitation pulse is frequency-modulated using the gradient magnetic field offset and the frequency offset which were obtained by shimming, then also on each collected set of echo data, a similar phase shift according to the frequency modulation occurs while the echo is read out. Accordingly, if an image is reconstructed from the echo data which is obtained by the EPI method with frequency correction using the frequency offset, an image under the influence of the phase shift is obtained. Which is to say, the frequency offset which was obtained by shimming is used with the gradient magnetic field offset. Furthermore, the frequency offset can be used not only for modulation of an excitation pulse and but also for frequency modulation that is needed while the echo is read out.
Especially, in the case of a scan by SS SE EPI (Single Shot Spin Echo Planar Imaging) method, there is a problem that influence of the phase shift on the echo data comes out noticeably on an image. In imaging under SS SE EPI method, a refocus pulse is impressed following impression of an excitation pulse and a lot of echo data are collected sequentially on a section of collecting data of SE (Spin Echo) method to collect echo data by inversion of the gradient magnetic field, thereby imaging one image by one excitation. Because of this, in imaging by the SS SE EPI method, every time echo data is collected, a shift amount the same as the frequency shift of the excitation pulse becomes a phase shift and is overlayed onto the echo data while echo data is read out. Accordingly, as the later echo data is collected, the larger phase shift exists thereon. Further, the phase shift that overlays onto the echo data appears as a positional shift on image data after Fourier transform.
FIG. 16 is a diagram showing one example of an image which was obtained by a scan under the conventional SS SE EPI method.
FIG. 16 is a sagittal cross-section image of a cylindroid of copper sulfate bottle phantom. This sagittal cross-section image was created by the way that multiple axial cross-section images which are included in an area having about 15 cm width in the axial direction are acquired by the SS SE EPI sequence and MPR multi-planar reconstruction) processing is performed to the acquired axial cross-section images.
On the image shown in FIG. 16, image distortion by a position shift of image data due to influence of frequency modulation on an excitation pulse is observed. That is to say, lateral ends of the copper sulfate bottle phantom shift upward.
Such an image position shift results from a phase shift while echo data is read out which occurs at a frequency corresponding to the frequency offset which was used for an excitation pulse for the frequency correction. This is a phenomenon which can happen to all imaging methods commonly used to collect multiple echo data sequentially after excitation as well as the SS SE EPI method.
Furthermore, in the case of generating a single image by combining multiple images (e.g., imaging with moving a bed), there is a case that a step discontinuity occurs on connection of parts of images each other and discontinuity occurs to the images if a position shift exists on each image serving as an object to be combined.